This invention relates to a novel method for efficient recovery of motive power (electric power) from the sensible heat of the medium- to low-temperature industrial waste gas. The fundamental idea of this invention resides, as illustrated in FIG. 1, in causing the main cycle in its steady state to complete one cycle of operation through the action of the steam in a high-pressure semi-sealed state upon the superheated steam zone by, first of all, bringing the steam in the dry saturated state 1" of the pressure p.sub.1 to the state 2 of the pressure p.sub.2 by means of adiabatic compression, bringing the steam in that state to the state 2 by means of constant-pressure expansion by the externally supplied heat Q.sub.2 and then bringing the steam in that state to the pressure p.sub.1 by means of adiabatic expansion within the high-pressure stream turbine. And the expanded steam is divided into the main-cycle circulation steam and the exhaust steam. The condensed cold water (described more fully afterward) of the vacuum pressure p.sub.0 is elevated to p.sub.2 (&gt;p.sub.1) by a pump and is sprayed into the circulation steam, thereby allowing the circulation steam to be cooled by the cooling action (preponderantly owing to the latent heat caused by the vaporization of the condensed cold water) and bringing the circulation steam and the injected water simultaneously to the saturated state 1". In this case, the mass flow of the circulation steam and that of the injected water is identically fixed so that the mass flow of the total resultant steam will equal that of the steam starting from the main cycle state 1". Subsequently the residual exhaust steam from the main cycle is caused to be adiabatically expanded in the auxiliary cycle to the vacuum pressure p.sub.0 by means of the low-pressure steam turbine and then condensed to a liquid state by means of the condenser. Thus a total cycle enabling the main cycle and the auxiliary cycle to be joined to each other is established. With the resultant condensed cold water is to be used for the purpose of spray to cool the circulation steam in the main cycle so that the mass flow of the injected water must be equal to that of the exhaust steam in light of the material balance.
It is clear from the description given above that the [(superheater+steam turbine)] system formed by omitting the vaporizer from the motive power recovery system which combines the conventional boiler (vaporizer+superheater) and the steam turbine constitutes the basic feature of the present invention. When the pressures p.sub.1 and p.sub.2 indicted in the diagram of FIG. 1 are approximated to each other and finally equalized (p.sub.1 =p.sub.2), then the circulation steam in the main cycle simply flows through its course while performing absolutely no work. In this case, therefore, the so-called main cycle functions as a simple circulation cycle which forms a combination of superheater and steam generator. Comparison of this specific, novel cycle with the cycle of the conventional waste-heat boiler reveals that the efficiency of the novel cycle which recovers the sensible heat of a waste gas and converts it into motive power is considerably higher than the efficiency of the cycle of the conventional waste-heat boiler.